"went back later with some fellow divers, going down 15 metres" At 15 metres, the pressure is 2.5 × the surface pressure. Pressure at the surface is 1 bar, an

"went back later with some fellow divers, going down 15 metres" At 15 metres, the pressure is 2.5 × the surface pressure. Pressure at the surface is 1 bar, and fresh water pressure increases at 1 bar per 10 metres. 15 metres = 2.5 bar